A multilayer wiring board in which insulating layers and conductive layers (wiring conductors) are layered in an alternate manner has been known as an example of a wiring board for mounting a semiconductor element such as a semiconductor integrated circuit element. In such a multilayer wiring board, a via hole or a through-hole passing through one or more insulating layers in a thickness direction thereof is formed to electrically connect one conductive layer to another conductive layer.
In a manufacturing process of the multilayer wiring board, an upper resin insulating layer 63, which contains a conductive layer (wiring conductor) 62 as illustrated in FIG. 2(a), is formed on a lower resin insulating layer 61. Next, laser machining is performed to remove parts of the resin insulating layer 63 and the conductive layer 62, thus forming a via hole 65 or a through-hole as illustrated in FIG. 2(b). When the via hole 65 or the through-hole is formed, smear (residue) S, resulting from the material of the upper resin insulating layer 63, remains on an inner side of the via hole 65 or in the vicinity thereof. If the smear S is left unremoved, the formation of the conductive layer 62 on the inner side of the via hole 65 may fail to establish sufficient electrical conduction by the conductive layer 62. Thus, a desmear treatment for removing the smear S is performed to obtain a state in which the upper surface of the conductive layer 62 is exposed as illustrated in FIG. 2(c).
A wet desmear treatment method has been known in the art as an example of desmear treatment methods for wiring board materials.
The wet desmear treatment method is a method for removing smear remaining in a wiring board material by immersing the wiring board material into an alkaline solution, which is prepared by dissolving potassium permanganate or sodium hydroxide, to dissolve or peel off the smear (see Patent Literature 1, for example).
The wet desmear treatment method, however, has a problem of a considerably high desmear treatment cost. The reasons for this include lowered throughput due to the need to clean and neutralize the wiring board material after being immersed into the alkaline solution, the need to perform liquid waste disposal of the used alkaline solution, etc.
Moreover, with an increasing demand for finer wiring patterns in wiring boards in recent years, via holes with smaller diameters have been required to be formed. When the desmear treatment is performed on a wiring board material having a via hole with a small diameter, the alkaline solution fails to sufficiently penetrate into the via hole. Thus, it is difficult to perform the required desmear treatment reliably.
According to a dry desmear treatment method, in contrast, it can be expected to perform the desmear treatment in a short amount of time. Additionally, cost for the desmear treatment can be reduced since there is no need to perform the cleaning and neutralization of a wiring board material and the liquid waste disposal. Furthermore, the dry desmear treatment method can be used for a wiring board material having a via hole with a small diameter (see Patent Literature 2, for example).
As an example of the dry desmear treatment method, may be mentioned a method according to which a wiring board material is irradiated with vacuum ultraviolet rays with a wavelength of not higher than 200 nm under an atmosphere in which oxygen exists to decompose and remove smear. With a method utilizing active oxygen, such as ozone, produced by vacuum ultraviolet rays emitted from an excimer lamp, in particular, it can be expected to perform the predetermined treatment more efficiently and in a short amount of time.